1. Field of Invention
The present invention relates to an apparatus and method of measuring effective porosity using radon, and more particularly, to an apparatus and method of measuring effective porosity of various media such as rock or soil using radon that is an inert gas.
2. Description of the Prior Art
The porosity of rock, which refers to a capability of storing fluid, provides a basis for determining economic feasibility of petroleum, natural gas, subsurface water, or mineral resource and is used in a variety of fields other than that, and thus, it is very important to accurately calculate the porosity.
For example, recently, studies on underground storage of carbon dioxide for solving global warming are actively carried out, and an investigation for selecting proposed sites for storing carbon dioxide is performed along therewith. In order to find an underground storage place of carbon dioxide in a commercialized scale, evaluations for a scale of a quantitative storage space and a storage capacity should be carried out. To this end, it is preferentially required to find a stratum having a porosity of at least 8%.
In addition, weathering and permeability of rock plays an important role in terms of stability securement and long-term management of a bedrock structure and disposal of radioactive waste in subterranean caves. The weathering and permeability of rock is greatly influenced depending on internal structure properties of the rock. That is, the weathering may proceed rapidly depending on the quantity of pores, fine cracks and the like inside rock. In addition, a quantitative evaluation of an internal structure of rock may be a means capable of quantitatively evaluating the degree of weathering of the rock. Therefore, in terms of long-term management of a bedrock structure, it is very important to accurately understand the internal structure of rock in three dimensions.
Carbonate rock is very important reservoir rock, and the understanding of lithological properties of carbonate rock is very important in evaluating economic feasibility of a mining area and oil deposits. Particularly, techniques of analyzing and predicting permeability of carbonate reservoir rock may be helpfully used in greatly reducing capital risk when developing a mining area. The permeability of carbonate reservoir rock is mainly influenced by porosity of reservoir rock, connectivity between pores, temperature of reservoir rock, precipitation of asphaltene and the like. However, it has been known that among the factors, the porosity of reservoir rock most greatly influences the permeability of reservoir rock.
The liquid substances in the ground permeate through pores connected between soil particles constituting the ground. In addition, it has been known that the liquid substances in bedrock moves depending on cracks and fine cracks formed by weathering, fault activity, discontinuity surface, joint, and the like. Therefore, all spaces connected to one another that relate to a mechanism of such movement may be represented as spatial meaning of the effective porosity. The effective porosity is one of the very important parameters capable of estimating a contaminant penetration pathway, the subsurface water content, and the like.
Even in order to evaluate the subsurface water content in the ground for rain and the inflow of contaminants leaked from the ground surface, it is preferentially necessary to understand porosity and effective porosity, which are physical properties of the ground. Further, in the case of subsurface dam in the course of construction in an alluvium region that is an unconsolidated, unconfined aquifer for the purpose of emerging conservation and effective use of water resources, in order to estimate storage capability of subsurface water in the target area, the measurement of porosity and effective porosity of the ground should be preceded.
The porosity may be classified into absolute porosity or total porosity and effective porosity depending on whether or not geotechnically isolated pores are included. The absolute porosity is a ratio of the volume of all empty spaces in a sample to the total volume thereof with or without connectivity between the spaces. On the contrary, the effective porosity is defined as a ratio of the total volume of the pores connected to one another allowing fluid to pass, except isolated pores, to the total volume.
Indoor measurement of porosity of rock is generally performed using a saturation method. According to the definition of porosity, it can be seen that the porosity calculated by a saturation method is effective porosity. The volume of pores is calculated using a difference in weight between a saturated state and a dried state of rock according to the saturation method. Thus, the accuracy in measuring the porosity depends on whether rock is saturated 100%. In Korean Society for Rock Mechanics and International Society for Rock Mechanics, a saturation method using vacuum is employed as a standard test method, in which a test specimen is immersed in water in a vacuum state below 800 Pa (6 torr) for one or more hours to saturate the test specimen.
Here, when a vacuum pump with low pumping rate is used or a plurality of test specimens are saturated in a lump, the test specimens should be immersed in water for a long time in a vacuum state. Particularly, if a test specimen contains substances soluble in water, a surface-dried water saturation weight is reduced. In addition, there may be various disadvantages according to the vacuumization by the water-immersion. One of the disadvantages is air bubbles captured on the surface of the test specimen when it is immersed in water and saturated. In the standard test, the test specimen is periodically disturbed in order to remove the air bubbles, which is practically extremely difficult operation. Moreover, vacuum efficiency is deteriorated and influenced even by the amount of water because of water serving as buffer along with the air.